A new mode for WebAuthn that displays a credential selection UI only if the user has a discoverable credential registered with the Relying Party on their authenticator. The credential is displayed alongside autofilled passwords. This solves the bootstrapping problem when replacing traditional username and password with WebAuthn: websites can fire a WebAuthn call while showing a regular password prompt without worrying about showing a modal dialog error if the device lacks appropriate credentials.
WebAuthn is designed to make it impossible to query for credential availability without going through a complete, modal dialog heavy, authentication flow. If a website doesn't know whether a registered authenticator is available on the device the user is attempting to log-in, it can't risk showing the modal dialog since it can fail right away and disrupt the user. Conditional UI solves this by showing UI only if we know in advance that the user has appropriate credentials available on their current device. The credentials are shown alongside autofilled passwords to aid in the transition from passwords to WebAuthn credentials ("passkeys").
Docs: https://docs.google.com/document/d/1KzEWP0aoLMZ0asfw6d3-7UHJ6csTtxLA478EgptCvkk
Samples: https://webauthn-conditional-ui-demo.glitch.me
Explainers: https://github.com/w3c/webauthn/wiki/Explainer:-WebAuthn-Conditional-UI